This work continues the application of stable isotope micromethods and selected ion monitoring GCMS to study metabolic fuel transport in infants, children, and adults without the hazard of radiotracers. Using the established continuous intravenous infusion of isotope technique, this project has measured carbohydrate, amino acid, and lipid flux in animals and man with deuterium, carbon-13, and nitrogen-15 labeled substrates. The results of these studies have provided the first systematic quantitative estimation of glucose production in infants and children, the first direct demonstration of gluconeogenesis from alanine in the newborn within the first 8 hours of life, the first estimation of carbon recycling as a source of neonatal hepatic glucose output, and the first extensive studies of the effects of protein and amino acid perturbations on the flux of two essential amino acids in man, lysine and leucine. Continuation of these investigations will allow further delineation of the sources of hepatic glucose output in the human neonate under normal circumstances in those conditions known to affect glucose homeostasis in the newborn. Additional studies employing measurement of 13CO2 excretion following administration of carbon-13 labeled leucine will allow compartmentalization of leucine metabolism, and estimation of protein synthesis and catabolism using this technique in man. Finally, methodology will be developed to allow measurement of plasma 3-methylhistidine and the ketoacids of branched chain amino acids.